The main problems encountered during the development of rechargeable lithium batteries are: problems of safety and gradual degradation of performance (premature failure). These problems result from an unsuitable mechanism of deposition of the lithium during the charging process and from the deposition of the lithium into the cathode during cell reversal at the end of the discharge. Lithium which is deposited on the anodic current collector during charging and which is deposited on the cathode, is in the form of a fine powder or in the form of dendrites. This high surface are a lithium, is pyrophoric and has a sensitivity to shock and heating in the environment of the non-aqueous cell. Thus there occur explosions or venting under thermal shocks or when such cell is subjected to a mechanical shock. The lithium dendrites are likely to form internal shorts with sparks which can result in a run-away reaction.
The phenomenon of cell-reversal is more likely to occur in multi-cell batteries where the weakest of the cells may turn into reversal at the end of the discharge of the battery.
One of the reasons for gradual cell degradation with repetition of cycles is to be found in the formation of a passivating layer which is formed on the surface of the lithium anode, which acts as a solid electrolyte interphase (termed SEI).
When a non-aqueous lithium cell is charged, lithium is plated out through such surface layer which consists of a polycrystalline material and thus is non-uniform. The results is a preferred deposit of the lithium at the grain boundaries of the microcrystals, forming dendrites or spongy deposits. During discharge part of such lithium particles lose electrical contact with the current collector and thus are not utilized, resulting in a Faradic efficiency of less than 100 per cent. Upon repeated charge-discharge cycles of such a battery, anodic mass is lost and ultimately leads to a premature failure of the battery. The lithium deposits are characterized by their high surface area, resulting in enhanced electrolytic degradation, resulting in decrease of performance.
It is one of the main aims of the present invention to modify the mechanism of lithium deposition in non-aqueous lithium batteries, resulting in a safe electrochemical cell which is repeatedly rechargeable and which has a very slow rate of degradation during repeated cycles.